1. Field of the Invention
The present invention relates to a combustion condition detection apparatus for an internal combustion engine for sensing at least the occurrence of a misfire or a knock in an internal combustion engine by detecting a change in the amount of ions generated by combustion in the internal combustion engine.
2. Description of the Related Art
It is generally known that ions are generated when a fuel is combusted in a cylinder of an internal combustion engine. If a probe to which a high voltage is applied is placed in the cylinder, the generated ions can be observed as an ion current.
When a knock occurs in the internal combustion engine, a vibration component due to the knock is superimposed on the ion current. Therefore it is possible to detect the occurrence of a knock by extracting such a vibration component.
A conventional knock detection apparatus will be described with reference to drawings. FIG. 9 is a diagram showing the structure of a conventional combustion condition detecting apparatus for an internal combustion engine by using ion current, e.g., the one disclosed in Japanese Patent Laid-Open No. 2001-140740. FIG. 10 is a diagram showing details of the structure of an ion current detector in the conventional combustion condition detecting apparatus for an internal combustion engine. FIG. 11 is a timing chart for explaining the operation of the conventional knock detection apparatus.
Referring to FIG. 9, an ignition plug 1 is used as a probe for detecting an ion current. A bias means 3 is charged at a high voltage (bias voltage) for detection of ion current by using a secondary voltage of an ignition coil 2. After the completion of discharge for ignition, the bias voltage of charge accumulated in the bias means 3 during the discharge period is applied to the terminal of the ignition plug 1 to enable an ion current to be detected by an ion current detector 4 connected to an engine control unit (ECU) 5.
Referring to FIG. 10, when ion current 3s is input by the high voltage applied by the bias means 3, it is distributed by a current distributor 6 in the ion current detector 4 to a band pass filter (BPF) 7 for extracting a knock vibration component and a comparison section 8 for determination of a combusting condition. This comparison section 8 determines that combustion is being effected and outputs a pulse to the ECU 5 if the input is larger than a predetermined combustion pulse threshold value 8a. From this pulse, a combusting/misfiring condition can be determined. This pulse will hereinafter be referred to as combustion pulse 8s. 
A knock vibration component is extracted by the BPF 7 and then amplified by an amplifier 9. A comparison section 10 for determination of a knock determines that there is a knock if the vibration component is larger than a predetermined knock detection threshold value 10a, and then an output section 11 outputs a knock pulse 11s to the ECU 5.
Japanese Patent Laid-Open No. 2001-073862 discloses a knock detection apparatus which has an integration circuit for integrating (charging) a vibration component superimposed on an ion current, and a discharge circuit for discharging a predetermined amount of charge from the charge obtained as a result of the integration (charging), and which autonomously adjusts a knock detection threshold value according to discharge balance between the integration circuit and the discharge circuit.
Further, Japanese Patent Laid-Open No. 10-077944 discloses a knock detection apparatus described below. FIG. 12 is a diagram showing the structure of another conventional knock detection apparatus disclosed in Japanese Patent Laid-Open No. 10-077944.
When a knock occurs in an internal combustion engine, a knock signal at a particular frequency is superimposed on an ion current in a decreasing period after the ion current has peaked. As means for detecting a knock through ion current, therefore, a method of detecting only such a knock signal at a particular frequency while removing other signals (e.g., an LC resonance waveform) is preferably used. Therefore, it is preferable to provide a knocking window which is opened at a time after a time when unnecessary signals disappear and which is closed at a suitable time after a decrease in ion current (for example, at ATDC60°), and to detect a knock on the basis of an output from an ion current detection section during a time period through which the knocking window is open.
A “knocking detection method using ion current” has already been proposed (see JP 06-159129 A) in which a knock signal is separated from an output signal from this ion current detection section by using a band pass filter and is integrated and a knock is detected on the basis of the integrated value.
Referring to FIG. 12, an LC resonance waveform is removed from an output from an ion current detection section 19 by an LC resonance mask section 20 and the output from the ion current detection section 19 is thereafter input to a processing section 23 via a band pass filter section 21 and a peak hold (or integration) section 22. The operation of the peak hold section 22 is controlled through an window which is opened following a predetermined time after ignition according to the rotational speed of the internal combustion engine and the load on the internal combustion engine, and which is closed at a closing time corresponding to about 50°CA in terms of crank angle after opening. Noise components are removed by utilizing a phenomenon in which an integral value of noise assumed to be an instantaneous change in ion concentration increases in a stepping manner and a phenomenon in which an integral value of a knock signal increases continuously. It is well known that the LC resonance mask section 20 is provided between the ion current detection section 19 and the band pass filter section 21 for the purpose of eliminating the influence of LC resonance after discharge.
In the above-mentioned conventional knock detection apparatus disclosed in Japanese Patent Laid-Open No. 2001-140740, when energization of the ignition coil for ignition in another or the next cylinder is started, a false ion current 3sa is generated by electromagnetic induction caused by the start of energization of the ignition coil for ignition in the another or next cylinder, as shown in (c) of FIG. 11. False ion current 3sa appears as if combustion is effected or a knock occurs in the cylinder corresponding to the ignition coil through which ion current detection is being performed. A false knock component signal 9sa is generated as a knock component signal 9s by false ion current 3sa, as shown in (e) of FIG. 11. At this time, an erroneous knock pulse 11sa is generated as a knock pulse 11s, as shown in (f) of FIG. 11. Also, an erroneous combustion pulse 8sa is generated as a combustion pulse 8s, as shown in (d) of FIG. 11. Thus, there has been a problem in that there is a fear of erroneous combustion determination or erroneous knock determination.
The method of autonomously adjusting a knock detection threshold value according to discharge balance between an integration circuit and a discharge circuit has a problem in that even when no knock occurs, there is a fear of charging with a knock component signal generated by a false ion current, which increases the threshold value for detection of knocks, resulting in detection failure.
Further, the method of separating a knock signal from an output signal from the ion current detection section 19 by using the band pass filter section 21, integrating (holding the peak of) the separated knock signal, and detecting a knock on the basis of the integral (held peak) value has a problem in that a knock component signal generated by a false ion current cannot be removed by the band pass filter section 21, it is, therefore, impossible to discriminate the knock component signal generated by the false ion current and a knock from each other with reliability, and there is a fear of occurrence of a detection result erroneously indicating a knock.